An electrophotographic photoreceptor which has been practically used at present is generally classified into an inorganic photoreceptor using an inorganic photosensitive material and an organic photoreceptor using an organic photosensitive material.
As typical inorganic photosensitive materials, there are selenium photosensitive materials composed of amorphous selenium (a-Se), amorphous arsenic selenide (a-As.sub.2 Se.sub.2), etc., a photosensitive material composed of dye-sensitized zinc oxide (ZnO) or cadmium sulfide (CdS) dispersed in a binder resin; and a photosensitive material using amorphous silicon (a-Si).
However, the aforesaid inorganic photoreceptor using selenium series materials or CdS suffers problems in the heat resistance and the storage stability. Also, since these materials have a toxicity, there is a restriction that they cannot be simply wasted and must be recovered.
The ZnO-resin dispersion type photoreceptor has been scarcely used at present since it has a low sensitivity and a low durability.
Also, although the a-Si type photoreceptor has merits such as high sensitivity, high durability, etc., it has disadvantages that the production cost is high because of complexity in the production process thereof and image defects occur due by film defect specific to a-Si.
On the other hand, typical organic photoreceptors include those using a charge transferring complex of 2,4,7-trinitro-9-fluorenone (TNF) and poly-N-vinylcarbazole (PVK), and those of function separation type having a charge generating layer and a charge transport layer. Since there are many kinds of organic photosensitive materials for these organic photoreceptors, those free from the problems on the storage stability and toxicity can be produced by properly selecting the organic photosensitive materials, and besides they can be produced at a low cost. Further, durability thereof has been recently improved. Thus, the organic photosensitive materials are given attention as one of the most important photosensitive materials.
However, the organic photoreceptors using the aforesaid PVK-TNF charge transfer complex are still insufficient in sensitivity.
The aforesaid function separation type organic photoreceptors have a double layer structure of a layer containing a charge generating material capable of generating charge carriers upon irradiation with light (hereinafter, referred to as a charge generating layer) and a layer containing a material capable of receiving the charge carriers generated in the charge generating layer and transferring them (hereinafter, referred to as a charge transport layer), have a relatively excellent sensitivity, and are the main current of organic photoreceptors which are practically used at present.
When the charge generating material does not have a film-forming property by itself, it is used along with a binder resin, whereby the film is formed.
When a photosensitive layer is formed on a drum or a film by a coating method, the following properties are required:
(1) good electric characteristics, PA1 (2) a good dispersibility such that the components are not aggregated for improving the stability of the coating composition with the passage of time, PA1 (3) a uniformity of the coated surface for preventing the occurrence of roughening of the image quality, and PA1 (4) a good adhesion for imparting the mechanical durability as a photosensitive material.
However, when the aforesaid conventional electrophotographic photoreceptor having a photosensitive layer formed by dispersing a charge generating material in a binder resin is produced by a coating methods, the electrophotographic photoreceptor does not satisfy all the properties (1) to (4) and hence is not sufficient in electrophotographic characteristics.
Also, it is known that the dispersibility of a charge generating material in a photosensitive layer gives a large influence on the aforesaid properties (1) to (4) and hence it has hitherto been attempted to control the dispersibility of a charge generating material in a photosensitive layer. Hitherto, a termination time of the step of dispersing a charge generating material in a binder resin has been determined by monitoring the dispersion state of the charge generating material in a coating composition for a photosensitive layer using a centrifugal sedimentation type particle size distribution meter, a laser scattering type spectrophotometer, etc. as described in JP-A-63-136055 (the term "JP-A" means an unexamined published Japanese patent application), but the dispersion state monitored by the above conventional method does not exhibit good correlation with the electrophotographic characteristics of the resulting photosensitive layer and hence the conventional method is not a satisfactory method for determining a termination time of the step of dispersing a charge generating material.